The acceleration of rockets by the use of propellants is a well known technology. When propellants are used to accelerate vehicles into space, the rocket acceleration must be large compared to gravity (5 to 10 times g) so that impulse (force times time) is not wasted against gravitational force. When a rocket is in orbit or otherwise substantially uninfluenced by gravity the acceleration may be much smaller because gravity is no longer a limitation. Military rockets, on the other hand, must accelerate extremely rapidly, yet often the structure of the vehicle or the propellant composition limits the practical acceleration to a range of from 100 to 1000 g.
Rockets also find use in the rocket accelerated rod apparatus (RAR) such as is described in U.S. Pat. No. 3,509,821 for rapid penetration into dense media such as rock or metal. In RAR applications, an acceleration of several.times.10.sup.4 g is required if the rod is to be used for commercial applications of boring holes in rock or ground. The high acceleration is required so that the stand-off distance required for the rocket-rod to attain the required penetration velocity can be reasonably small, e.g. 10 meters. A typical velocity required for substantial penetration is approximately 2000 meters per second with the result that the acceleration (within a distance of about 10 meters) is approximately 20,000 g. Conversely, the time of acceleration of burning time of the propellant is very short, e.g. t=2 d/v.perspectiveto.10.sup.-2 seconds. Therefore there is a need for very fast burning propellants for rapid acceleration of projectiles for commercial uses.
In U.S. Pat. No. 3,616,855, which relates to the bulking and caving of underground ore bodies, a solid propellant is used to heave the ground after prestressing the formation by injecting an appropriate settable propping material. In such applications of earth fracturing (which is a form of bulking) there also exists a need for particular propellant burn properties. As discussed in U.S. Pat. No. 3,616,855, the propellant should burn (i.e. form the bulking or fracturing gas) in a time that is a small multiple (e.g. 2 to 10 times) of the dynamic time of the system. In this regard, it is not desired to shock the formation because this compacts the rock and wastes energy that would otherwise be used to lift it and form fractures. Fracturing with detonating explosives have shown that the shock wave in general compacts the ground or rock and does not in general open new fractures. While a slower gas release is thus desired, too slow a release results in the gas or fluid being lost into the formation. Hence the gas should be released within a period of approximately 2 to 10 times the dynamic time of the system.
A typical case is a well 3500 feet (1 km) deep. The time for a compression wave to reach the surface and return, i.e. the dynamic time, is roughly 2 seconds for a formation having a sound speed of 1 km sec.sup.-1. Hence the gas release time from a preferred fracturing or bulking propellant should be 5 to 10 seconds.
The placement of the propellant will be within the bore hole, for example a bore hole 8 inches in diameter and 1200 feet or 300 meters in length. The propellant must burn a length of 300 meters in 20 seconds, or a burn velocity of 15 meter sec.sup.-1. This also is in the range of burn velocity that is the objective of the present invention, but not available in conventional propellants.
There are thus two circumstances where a fast burning propellant is needed for useful purposes: fast rocket acceleration and underground well fracturing. In both cases the burn rate and hence the minimum pressure of the burning gases is roughly the same, namely several hundred MPA or 10 to 20 thousand PSI. This higher pressure is the result of the mass flow times gas velocity, or the time rate of change of momentum of the combustion gases. It is the useful pressure for either accelerating the rocket, or forcing the combustion gases into the rock for fracturing. Hence the high pressure of combustion is a necessary and useful result of a fast burning propellant. The magnitude of the pressure is determined by the geometry or confinement of the burn. It is this geometry or confinement that leads to two different mechanisms of fast burning propellants of this disclosure.